Polyimides are known in the art for use in the manufacture of integrated circuits including chips (e.g. chip back end of line), thin film packages and printed circuit boards. Polyimides are useful in forming dielectric interlayers, passivation layers, alpha particle barriers and stress buffers. Photosensitive polyimides are particularly useful as an interlayer dielectric material to insulate the conductor wiring interconnecting the chips on a multichip module. This is known as thin-film wiring. A multichip module s an intermediate level of packaging between the chips and the circuit board. Multichip modules are generally known in the art. Multichip modules are made up of multiple layers of power, signal and ground planes which deliver power to the chips and distribute the input/output signals between chips on the module or to/from the circuit board.
Photosensitive polyimides can also be used as the dielectric material for insulating the conductor wiring on a silicon carrier as a multichip module. The use of photosensitive polyimide provides a simplified method for the fabrication of the patterned polyimide required for these thin film wiring schemes. It is important that the final properties of the patterned polyimide are not significantly altered relative to the nonphotosensitive polyimides also used in forming the multichip module.
Photosensitive polyimides useful as a dielectric interlayer for conductor wiring in a multichip module are known in the art. "Polyimide Coatings" by Craig, Electronic Materials Handbook, Vol. 1, discloses negative tone photosensitive polyamic acid esters where the ester moiety is covalently bound to the polymer backbone and contains unsaturated linkages in the form of acrylates or methacrylates. Upon exposure to light, these groups, in conjunction with a suitable photopackage (photoinitiator and sensitizer), crosslink via a free radical mechanism resulting in differential solubility between the exposed and unexposed regions. Subsequent development with a suitable solvent system followed by an appropriate cure results in a negative tone imaged polyimide dielectric interlayer. However, during the cure, the components of the photopackage in the unexposed area are volatilized resulting in excessive shrinkage of the polyimide interlayer.
Yoda et al., "New Photosensitive High Temperature Polymers for Electronic Applications," J. Macromol. Sci.-Chem., A21(13&14), 1641 (1984), discloses a similar approach which involves incorporating an ammonium salt as a photosensitive group. The ammonium salt is formed from the reaction of a suitable poly(amic acid) with a tertiary amine having unsaturated groups in the form of acrylates or methacrylates. However, again the components of the photopackage in the unexposed area are volatilized during the cure of the polyimide generally resulting in excessive shrinkage of the polyimide film.
J. Pfeiffer et al., "Direct Photoimaging of Fully Imidized Solvent-Soluble Polyimides," Proc. Second Intern. Conference on Polyimides, Ellenville, N.Y. (1985), discloses a photosensitive polyimide based on fully imidized polyimide having groups which crosslink upon exposure to light. Upon exposure to light, the crosslinking reaction introduces differential solubility in the polyimide film which can then be developed with organic solvents. Although fully imidized photosensitive polyimide systems have less shrinkage of the developed pattern, they do experience shortcomings in the area of thermal stability, mechanical properties and solvent induced stress-cracking due to the soluble nature of the polyimides. There still is a need in the art for a suitable photosensitive polyimide system for use in making integrated circuits.
It is, therefore, an object of the present invention to provide photosensitive polyimide system for use in making integrated circuits.
Other objects and advantages will become apparent from the following disclosure.